Currently, the most common route to produce phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone.
Another process involves the hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513.
In some embodiments, cyclohexanone is converted via dehydrogenation to additional phenol (see International Patent Publication No. WO2010/024975). Such a dehydrogenation step is generally achieved by contacting the cyclohexanone with a supported noble metal catalyst at a temperature of about 250° C. to about 500° C.
For example, U.S. Pat. No. 3,534,110 discloses a process for the catalytic dehydrogenation of cyclohexanone and/or cyclohexanol to phenol over a catalyst comprising platinum and preferably iridium on a silica support. The catalyst also contains 0.5 to 3 wt % of an alkali or alkaline earth metal compound, which, according to column 3, lines 43 to 49, should be incorporated after addition of the platinum since otherwise the resulting catalyst composition has inferior activity, selectivity, and life.
In addition, U.S. Pat. No. 3,580,970 discloses a process for the dehydrogenation of cycloaliphatic alcohols and ketones to the corresponding hydroxyaromatic alcohols in the presence of a catalyst comprising a Group VIII metal, particularly nickel, and tin in a molar amount of about 1.7 to about 15 moles of Group VIII metal per mole of tin. The catalyst may further comprise an alkali metal stabilizing agent in an amount between about 0.3 to about 10 parts by weight of an alkali metal sulfate per part by weight of the Group VIII metal.
U.S. Pat. No. 4,933,507 discloses a method of dehydrogenating cyclohexenone to phenol comprising reacting hydrogen and cyclohexenone in the vapor phase in a molar ratio of 0.5 to 4.0 moles of hydrogen per mole of cyclohexenone at a pressure of at least one atmosphere and a reaction temperature of 300° C. to 500° C. using a solid phase catalyst containing platinum, in the range of 0.2 to 10 wt % of the sum of the catalyst plus support, and an alkali metal, in the range of 0.2 to 3.0 calculated in terms of the weight ratio of K2CO3 to platinum, both the platinum and the alkali metal being carried on a support.
U.S. Pat. No. 7,285,685 discloses a process for the dehydrogenation of a saturated carbonyl compound, such as cyclohexanone, in the gas phase over a heterogeneous dehydrogenation catalyst comprising platinum and/or palladium and tin on an oxidic support, such as zirconium dioxide and/or silicon dioxide. In general, the dehydrogenation catalyst contains from 0.01 to 2 wt %, preferably from 0.1 to 1 wt %, particularly preferably from 0.2 to 0.6 wt %, of palladium and/or platinum and from 0.01 to 10 wt %, preferably from 0.2 to 2 wt %, particularly preferably from 0.4 to 1 wt %, of tin, based on the total weight of the dehydrogenation catalyst. In addition, the dehydrogenation catalyst can further comprise one or more elements of Groups I and/or II, preferably potassium and/or cesium, in an amount of from 0 to 20 wt %, preferably from 0.1 to 10 wt %, particularly preferably from 0.2 to 1.0 wt %, based on the total weight of the catalyst.
Research into metal-containing cyclohexanone dehydrogenation catalysts has now shown that, although potassium plays a positive role in improving the stability of the dehydrogenation metal, depending on the amount of potassium present, potassium can also have an adverse effect on the phenol selectivity of the catalyst by increasing the formation of unwanted by-products. Surprisingly, however, it has been found that by controlling the potassium content within very narrow limits, between 0.15 and 0.6 wt %, it is possible to achieve optimal phenol selectivity while retaining enhanced stability of the dehydrogenation metal.